Microscopes have improved over the years to permit ever greater sensitivity and precision, yet the focal spot diffraction limit of optical lenses has ot been broken until very recently. Stimulated Emission Depletion (STED) technology is a physical structuring of the illumination focus to produce effective illumination points 3-4 times smaller than the diffraction limit. Just as confocal microscopy provides 2-3 times the resolution of standard microscopy, STED superresolution outperforms confocal microscopy by at least this margin and closer approaches the resolution of the electron microscope. The Integrated Light Microscopy Facility (ILMF) at The University of Chicago serves more than 160 NIH-funded researchers in the Biological Sciences Division, as well as interdisciplinary researchers from the Physical Sciences Division and neighboring Institutions. During the past decade the ILMF has witnessed an explosive growth in the use of confocal microscopy. Particularly popular is the most advanced instrument in the ILMF: a Leica SP5 multiphoton confocal microscope, which is typically booked to capacity and must be reserved far in advance. The Leica SP5 II STED-CW microscope requested in this proposal is an enhanced version of the SP5 that includes greater sensitivity, and, most significantly, provides real-time superresolution capability. The instrument includes the (newer) continuous-wave depletion laser scheme that is better suited for live-cell probes. The system includes the latest technology, a hybrid detector that offers APD-like sensitivity with lower noise and wider dynamic range. The researchers at the University of Chicago require the resolution enhancement as well as the increased capacity, and have undertaken a lease to obtain these tools while replacing a failing, older SP2 confocal microscope in a Core serving 162 PIs and over 750 users, an action that demonstrates strong Institutional Commitment for this proposal. The seamlessly integrated STED capability is a major technical advance for researchers at The University of Chicago, and is a novel regional resource also available to researchers from other Institutions. PUBLIC HEALTH RELEVANCE: This request is to buyout an interim lease that the University of Chicago has undertaken to provide continued basic and newly-advanced high-end research microscopy, replacing a failing older system. The requested superresolution microscope has undergone extensive in-house testing, is housed in an established Core Facility, and provides novel capabilities that will significantly enhance the research programs of a large NIHfunded user community.